Telephone network protective coupler

ABSTRACT

A telephone network protective coupler for connecting terminal equipment to a telephone central office by means of a telephone line. The coupler includes two hybrid circuits which are interconnected to form a transmitting path and a receiving path. A power limiter in the transmitting path can be programmed remotely by selection of an external programming resistor without the direct introduction of noise into the transmitting path. The programming feature permits the gain of the transmitting portion of the device to be varied in steps from 0 to -12dB., and the maximum power output to be varied correspondingly in steps from 0 to -12 dBm. in order to arrive at an optimum signal power level of -12dBm. at the central office. The power limiter includes a controller gain amplifier for varying the attenuation of the transmitting path to a.c. signals, a remotely adjustable attenuator connected to a d.c. voltage source, and a gain control circuit connected to the amplifier and to the attenuator and responsive to adjustment of the attenuator for controling the gain of the amplifier. The power limiter further includes a rectifier for rectifying the a.c. signals after passage through the amplifier, and a maximum power control circuit connected to the rectifier and the attenuator and responsive to the adjustment of the attenuator for keeping the ratio of the amplitude of the rectified a.c. signals to the output voltage of the attenuator less than or substantially equal to a fixed constant.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to couplers for interfacing the telephone networkwith terminal equipment and more particularly to such couplersincorporating signal power limiting. The invention further concerns aprocess for limiting the signal power transmitted to the telephonenetwork by terminal equipment interfaced therewith.

2. Description of the Prior Art

Federal Communications Commission Rules Part 68 entitled "Connection ofTerminal Equipment to the Telephone Network" provide uniform standardsfor the protection of the telephone network from harm caused by theconnection of terminal equipment thereto. When a customer desires toconnect terminal equipment which has been registered, he is required tonotify the telephone company of each telephone line to which he intendsto connect such equipment. The telephone company, after determining theattenuation of each such telephone line between the interface and thetelephone company central office, will make such connections as arenecessary in each so-called programmed data jack which it will installat the interface, so as to allow the maximum signal power delivered bysuch terminal equipment to the telephone company central office toreach, but not exceed, the maximum allowable signal power permitted atthe telephone company central office.

The programmed data jack incorporates a programming resistor. The properprogramming resistor (Rp) is to be selected by the telephone company atthe time of installation based upon the loop loss of the telephone lineto program the signal power output of the terminal equipment so as toarrive at an optimum signal power level of -12 dBm at the centraloffice. Table 1 reproduced from the Federal Register Vol. 41, No. 134(July 12, 1976) gives the required resistance value for the programmingresistor for each value of the desired signal power level in dBm.

                  TABLE 1                                                         ______________________________________                                                           Programmed Data                                                               Equipment                                                  Programming Resistor (Rp)                                                                        Signal Power Output                                        ______________________________________                                        short              0          dBm                                               150 ohms         -1         dBm                                               336 ohms         -2         dBm                                               569 ohms         -3         dBm                                               866 ohms         -4         dBm                                             1,240 ohms         -5         dBm                                             1,780 ohms         -6         dBm                                             2,520 ohms         -7         dBm                                             3,610 ohms         -8         dBm                                             5,490 ohms         -9         dBm                                             9,200 ohms         -10        dBm                                             19,800 ohms        -11        dBm                                             open               -12        dBm                                             ______________________________________                                    

Terminal couplers are known which provide means wherebycustomer-provided automatic terminal equipment may be connected to theswitched telecommunications network for data and voice communications.One such coupler is manufactured by the Elgin Electronics Inc. as theEDC 1001 A Automatic Data Coupler and described in Elgin System PracticeBulletin 20011 (July 1972), Issue 3. When the connections are completed,the data coupler must be adjusted internally to limit the customersignal power to a level which will not exceed a -12 dBm signal level atthe serving central office.

The prior art connected the programming resistor directly into thetelephone line as apart of an attenuator. The disadvantage with thisarrangement is that with the resistor in the signal line bidirectionalattenuation results, that is, both the received and transmitted signalsare attenuated. Moreover, any noise induced into the attenuator leadswill be added to the throughgoing signal.

No terminal coupler is known where the programmable resistor controlsonly the transmitted signal and/or the programming resistor is not partof the signal circuit.

SUMMARY OF THE INVENTION

It is therefore one object of the present invention to provide atelephone network protective coupler for connecting terminal equipmentto a central office by means of a telephone line which can be remotelyadjusted to limit the transmitted signal power on the line.

It is another object of the present invention to provide a telephonenetwork protective coupler for connecting terminal equipment to acentral office by means of a telephone line which can be remotelyadjusted to control the transmitted signal level on the line.

It is a yet further object of the present invention to provide atelephone network protective coupler for connecting terminal equipmentto a central office by means of a telephone line which can be remotelyadjusted to limit the transmitted signal power and to control the signallevel without introducing noise into the signal path.

It is yet another object of the present invention to provide an improvedmethod for limiting the signal power transmitted to a telephone networkcentral office by terminal equipment attached to a telephone line.

The objects of the present invention are achieved by a telephone networkprotective coupler for connecting terminal equipment to a central officeby means of a telephone line. The device includes two hybrids which areinterconnected to form a transmitting path and a receiving path. A powerlimiter in the transmitting path can be programmed remotely by selectionof an external programming resistor without the direct introduction ofnoise into the transmitting path. The programming feature permits thegain of the transmitting portion of the device to be varied in stepsfrom 0 to -12 dB., and the maximum power output to be variedcorrespondingly in steps from 0 to -12 dBm. in order to arrive at anoptimum signal power level of -12 dBm. at the central office. The powerlimiter includes a controlled gain amplifier for varying the attenuationof the transmitting path to a.c. signals, a remotely adjustableattenuator connected to a d.c. voltage source, and a gain controlcircuit connected to the amplifier and to the attenuator and responsiveto adjustment of the attenuator for controlling the gain of theamplifier. The power limiter further includes a rectifier for rectifyingthe a.c. signals after passage through the amplifier, and a maximumpower control circuit connected to the rectifier and the attenuator andresponsive to the adjustment of the attenuator for keeping the ratio ofthe amplitude of the rectified a.c. signals to the output voltage of theattenuator less than or substantially equal to a fixed constant.

In another aspect, the present invention involves a method of limitingthe signal power transmitted to a telephone network central office byterminal equipment attached to a telephone line. This method comprisesthe steps of providing a controlled gain amplifier for varying theattenuation of the transmitting path to a.c. signals, remotely adjustingan attenuator connected to a d.c. voltage source, and controlling thegain of the amplifier in response to the adjustment of the attenuator.The method further includes the steps of rectifying the a.c. signalsafter passage through the amplifier, and keeping the ratio of theamplitude of the rectified a.c. signals to the output voltage of theattenuator less than or substantially equal to a fixed constant inresponse to the adjustment of the attenuator.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a block diagram of a preferred embodiment of the telephonenetwork protective coupler of the invention.

FIG. 2 shows a schematic circuit diagram of the power limiter of thepreferred embodiment of the telephone network protective coupler of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts, and more particularly to FIG. 1thereof, there is shown a schematic block diagram of a telephone networkprotective coupler 11 for attaching subscriber provided terminalequipment to a telephone line. The terminal equipment and hence device11 is located at the subscriber station. Line 13 represents the incomingconnection from the terminal equipment. Line 15 represents the outgoingconnection from the coupler 11 via the subscriber loop to the centraloffice.

The coupler 11 is connected to and isolated from incoming line 13 andoutgoing line 15 through first and second hybrid circuits 17 and 19 alsocalled two-wire-four-wire terminating sets which are well known in theart. In telephone transmission circuits a hybrid circuit is a circuitfor interconnecting two-wire and four-wire circuits through adifferential balance or bridge circuit in which the two sides of thefour-wire circuit form the diagonals. Balancing circuit networks 21 and23, which are also known in the art, provide a compromise impedancematch to the impedance of lines 13 and 15 respectively. The two wireside terminals of the first hybrid circuit 17 are connected to theterminal equipment. The 4-wire side terminal pairs of the first hybridcircuit 17 are connected to the 4-wire side terminal pairs of the secondhybrid circuit 19 through conventional fixed gain amplifiers 25 and 27and a power limiter 29. The two wire side terminals of the second hybridcircuit 19 are connected to the conductors of the telephone line.Two-wire connections 31 and 33, respectively, represent the receivingpath for signals from the central office to the terminal equipment, andthe transmitting path for signals from the terminal equipment to thecentral office. The signal received by the terminal equipment is usuallyweaker than the transmitted signal and therefore it is inadvisable tosubject it to further attenuation.

The power limiter 29 is remotely controllable from the interface forlimiting the signal power delivered by the terminal equipment to thecentral office via the transmitting path.

As previously discussed, the signal power delivered by the terminalequipment to the central office must not exceed -12 dBm. The powerlimiter 29 is responsive to the selection of a remote programmingresistor for varying the gain of the transmitting portion of the devicein steps 1 dB from 0 to -12 dB as indicated by TAble 1 and the maximumpower output in steps from 0 to -12 dBm. to satisfy this requirement.

The power limiter 29 includes a controlled gain amplifier 35 connectedbetween the hybrid 17 and the fixed gain transmitting amplifier 27 forvarying the attenuation of the transmitting path to a.c. signals in thetelephone band which are typically 300-3500 Hz in frequency from theterminal equipment. A d.c. voltage source 65 supplies a reference inputto the controlled gain amplifier 35. In practice the gain of thecontrolled gain amplifier will be less than one. An attenuator 37 isconnected to a d.c. voltage source 39, and is programmed by adjustingthe value of an external programming resistor in a data jack at a remotelocation. The value of the programming resistor is selected by thetelephone company at the time of installation of the coupler 11 into thetelephone network. According to the value selected, the d.c. poweroutput from the attenuator 37 can be programmed for one of twelve stepsbetween 0 and -12 dB. The output of the attenuator 37 is passed througha noise filter 41 which removes noise coupled into the attenuator 37 viathe leads of the remote programming resistor. A gain control means 43 isconnected between the controlled gain amplifier 35 and the attenuator 37and is responsive to the programming of the attenuator for controllingthe a.c. gain G of the controlled gain amplifier.

The power loss in the transmitting path between the two hybrids can beexpressed by forming the algebraic sum of the losses in dB encounteredby the a.c. signals traversing it. This sum includes the controlledvariable loss in dB., 20 log G, of the controlled gain amplifier 35 plusa constant term in dB, 20 log c₁, where c₁ is a fixed constant,resulting from losses of the fixed gain transmitting amplifier 27 andthe hybrid circuits 17 and 19, and can be represented by formula A,namely

    20 log G+20 log c.sub.1 =P.sub.A                           (A)

p_(a) is the programmed attenuation of the coupler in dB. On the otherhand, the voltage loss of the attenuator 37 can be expressed in terms ofits attenuation constant k which is determined by the value of theprogramming resistor, and can be represented by formula B, namely

    20 log (k)=P.sub.A                                         (B)

Since P_(A) ranges between 0 and -12 dB, k has values between 1 and0.14. The gain control means 43 keeps the ratio of the a.c. gain G ofthe controlled gain amplifier 35 to the attenuation k of the attenuator37 equal to a fixed constant, according to the equality represented byformula C, namely

    (G/k)=(1/c.sub.1)                                          (C)

so that the power loss of the transmitting path as represented byformula A is matched to the power loss of the attenuator as representedby formula B.

In this manner, whatever step in the range of 0 and -12 dBm isprogrammed into the attenuator, the same step is remotely programmedinto the transmitting portion of the coupler 11. It will be readily seenthat this offers the advantage that the gain of the telephone networkprotective coupler 11 can be remotely controlled without the directintroduction of noise into the a.c. signal path via the leads of anexternal programming means.

Referring again to FIG. 1, the power limiter 29 further includes arectifier 45 connected to the output of the fixed gain transmittingamplifier 27 for rectifying the a.c. signals after passage through thecontrolled gain amplifier 35. A maximum power control means 47 receivesinputs from the rectifier 45 and the attenuator 37 via the noise filter41. The maximum power control means 47 is responsive to the programmingof the attenuator 37 for keeping the ratio of the amplitude of therectified a.c. signals V_(R) to the output voltage V_(A) of theattenuator less than or substantially equal to a fixed constantaccording to formula D, namely

    (V.sub.R /V.sub.A) ≦ c.sub.2                        (D)

the maximum power control means 47 overrides the gain control means 43when the input signals from the terminal equipment to the coupler 11exceed 1 milliwatt, and further reduces the gain of the controlledamplifier 35. The fixed constant c2 is chosen so that the amplitude ofthe rectified a.c. signals is limited to the value of the output voltageof the attenuator 37 for a 1 milliwatt input to the attenuator. In thismanner the output of the coupler is limited to a signal power equal tothe step in the range of 0 to -12dBm. programmed into the attenuator.

The primary embodiment of this invention is more particularly describedwith reference to FIG. 2 which shows a schematic circuit diagram of thepower limiter 29 of the telephone network protective coupler 11 of thisinvention. The circuits outlined in FIG. 2 with broken lines carry thesame reference numbers as the blocks in FIG. 1.

The attenuator 37 comprises a resistance network including resistor R₂and of the type described in the Federal Register Vol. 41, No. 134 whichcan be programmed to provide a signal output in 12 steps between 0 dB.and -12dB. The programming is accomplished by inserting an appropriateprogramming resistor across terminals 51 and 53 at the remote location.For purposes of example, when R1=R3=600Ω and R2=3600Ω Table 1 can beused for determining the correspondence between the selected programmingresistor and the power output of the resistance network. The attenuator37 is connected at its input to the negative terminal of the d.c.voltage source 39.

The output of the attenuator 37 feeds the noise filter 41 including aresistor R4 and a capacitor C1 which removes noise coupled into theattenuator via the leads of the programming resistor. The filteredsignal is then amplified in a buffer amplifier 55, polarity inverted andcoupled to the gain control means 43 via lead 57.

The controlled gain amplifier 35 can be a conventional type ofoperational amplifier 59 having resistors R7, R8, R9 and Rphrespectively as part of an a.c. gain G determining network and resistorsR8, R9, R10 and Rph respectively as a part of a d.c. gain g determiningnetwork thereof. The common feedback resistance can be computed byformula (E), namely

    R.sub.f =R.sub.9 + R.sub.8 Rph/(R.sub.8 + Rph)             (E)

the a.c. gain is represented by formula (F), namely

    G=R.sub.f /R.sub.7                                         (F)

the d.c. gain is represented by formula (G), namely

    g=R.sub.f /R.sub.10                                        (G)

a controlled resistance element in the form of a photoconductive cell 61is connected in parallel with the resistance R8 of the amplifier 59. Alamp 63 is focused on the photoconductive cell 61 to alter theresistance Rph of the cell. The inverting input terminal of theamplifier 59 is a.c. coupled to the transmitting path and connection 33via capacitor C2, and d.c. coupled to the positive pole of the battery65 via the resistor R10. The battery 65 also powers the lamp 63. Theoutput of the amplifier 59 is a.c. coupled to the fixed gaintransmitting amplifier 27 in the transmitting path via capacitor C3 andd.c. coupled to the gain control means via lead 69.

The gain control means 43 consists of a error amplifier 71 which feeds alamp driver Q1. The d.c. output voltages from the controlled gainamplifier 35 and the attenuator 37 are applied to the inverting inputterminal of a summing amplifier 73 through resistors R11 and R12respectively, which form a simple summing network. The combined (summed)input is integrated by the capacitor C4 in conjunction with the summingamplifier 73 to produce an output voltage which controls the operatingpoint of normally on lamp driver Q1 which is coupled to the controlledgain amplifier via lead 75. Lamp driver Q1 comprises an NPN transistorhaving its emitter connected to ground via resistor R13. Resistor R14provides a bias current for the lamp. The emitter of Q1 is also coupledto the inverting input terminal of the summing amplifier 73 via aresistor R15 and capacitor C5 to prevent loop oscillations. Zener diodeZ1 is provided to limit the range of variation of the current passingthrough lead 75 which controls the resistance Rph of the cell, and thusthe gain excursion.

The rectifier 45 is of conventional feedback diode design and serves torectify the a.c. signals after passage through the controlled gainamplifier 35 and the fixed gain transmitting amplifier 27. Its output iscoupled to the power control means 47 via lead 79.

The power control means 47 consists of another error amplifier 81feeding a lamp driver Q2. The d.c. output voltages from the rectifier 45and the attenuator 37 (via noise filter 41) are applied to the input ofthe summing amplifier 83 through resistors R19 and R20 forming a summingnetwork. The combined (summed) output is integrated by the capacitor C7in conjunction with the summing amplifier 83 to produce an outputvoltage which turns on normally cutoff lamp driver Q2 which is alsocoupled to the controlled gain amplifier 35 via lead 75. Lamp driver Q2comprises an NPN transistor having its emitter connected to ground viaresistor R23. The emitter of Q2 is also coupled to the inverting inputterminal of the summing amplifier 83 via a resistor R22 and capacitor C6to prevent loop oscillations. Zener diode Z2 is provided to limit therange of variation of the current passing through lead 75 which controlsthe resistance Rph of the cell, and thus the gain excursion.

In operation, the controlled gain amplifier 35 multiplies the constantvoltage of battery 65 by the d.c. gain g and applies it to the invertinginput terminal of the summing amplifier 73 through the resistor R11 as afirst input current. The attenuator 37 and the noise filter 41 multiplythe constant voltage of d.c. voltage source 39 by a factor proportionalto the attenuation constant k and also apply it to the inverting inputterminal of the summing amplifier 73 through resistor R12 as a secondinput current of opposite polarity to the first input current. As notedabove, the a.c. gain G and the d.c. gain g are intimately related byformulas F and G. Therefore, whenever the d.c. gain is changed, the a.c.gain experiences the same change in dB. The values of the resistors R7,R10, R11 and R12 are selected so that the two currents cancel when theequality represented by formula C is maintained, that is, when the ratioof the a.c. gain G of the controlled gain amplifier 35 to theattenuation factor k of the attenuator 37 is equal to a predeterminedfixed constant c₁ whose value is dependent upon the losses in the fixedgain amplifier 27 and the hybrid circuits 17 and 19. If the d.c. gain gis too low or too high, an error signal proportional to the differencein the two currents is applied to the base of the lamp driver Q1,shifting its operating point to vary the resistance of the controlledresistance element 61 until the a.c. gain G of the controlled gainamplifier 35 takes on a value to satisfy formula C. Thus, the erroramplifier 71 compares the gain of the controlled gain amplifier 35 tothe attenuation of the attenuator 37 and the lamp driver Q1 contols theresistance Rph of the controlled resistance element 61 to keep the ratioof the a.c. gain of the amplifier to the attenuation of the attenuatorequal to the fixed constant.

In the meantime, the rectifier 45 rectifies the voltage of the a.c.signals after passage through the controlled gain amplifier 35 and thefixed gain amplifier 27 to provide the voltage V_(R) and applies it tothe inverting input terminal of the summing amplifier 83 throughresistance R19 as a first input current. The attenuator 37 multipliesthe constant voltage of the d.c. voltage source 39 by the attenuationconstant k to provide the reference voltage V_(A) and apply it throughthe noise filter 41 to the inverting input terminal of the summingamplifier 83 through resistor R20 as a second input current. The valueof the resistors R19 and R20 are selected so that the two currentscancel when the equality represented by formula H is maintained, namely

    (V.sub.R /V.sub.A) =c.sub.2                                (H)

that is, when the amplitude (average value) of the rectified a.c.signals equals the value of the output voltage of the attenuator for a 1milliwatt input to the attenuator. If the input signals from theterminal equipment to the device 11 exceed 1 milliwatt, that is, if theinequality represented by formula I holds, namely

    (V.sub.R /V.sub.A) > c.sub.2                               (I)

an error signal proportional to the difference in the two currents turnson lamp driver Q2 which has been biased to cutoff. Q2 now overrides theaction of lamp driver Q1 and further reduces the resistance of thecontrolled resistance element 61 until the a.c. gain G of the controlledgain amplifier 35 takes on a value to satisfy formula D. Thus, the erroramplifier 83 compares the amplitude of the rectified a.c. signals to theoutput voltage of the attenuator, and the lamp driver Q2 controls theresistance of the controlled resistance element to keep the ratio of theamplitude of the rectified a.c. signals to the output voltage of theattenuator less than or substantially equal to a fixed constant.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. For example, onepossible variation is to use either a thermistor, a magnetoresistor, afield effect transistor, or a diode in place of the photoconductive cellas the controlled resistance element. It is therefore to be understoodthat within the scope of the appended claims, the invention may bepracticed otherwise than as specifically described.

What is claimed as new and desired to be secured by Letters Patent of the United States is:
 1. A telephone network protective coupler for connecting terminal equipment to a central office by means of a telephone line comprising:first and second hybrids; the 2-wire side terminal pair of the first hybrid adapted to be coupled to the terminal equipment and the 2-wire side terminal pair of the second hybrid adapted to be coupled to the conductors of the telephone line; the first terminal pair of the 4-wire side terminal pairs of the first hybrid being connected to the first terminal pair of the 4-wire side terminals of the second hybrid to form a receiving path for signals from the central office to the terminal equipment; the second terminal pair of the 4-wire side terminal pairs of the first hybrid being connected to the second terminal pair of the 4-wire side terminals of the second hybrid to form a transmitting path for signals from the terminal equipment to the central office: and means for limiting the signal power delivered by the terminal equipment to the central office; wherein the limiting means includes a controlled gain amplifier disposed in the transmitting path for varying the attenuation of a.c. signals on the transmitting path; an attenuator disposed externally of the transmitting path and having an attenuation set to a predetermined value; means for connecting the attenuator to a d.c. voltage source; a rectifier disposed externally of the transmitting path for rectifying the a.c. signals after passage through the amplifier; and maximum power control means connected to the rectifier and to the attenuator for keeping the ratio of the amplitude of the rectified a.c. signals to the output voltage of the attenuator less than or substantially equal to a fixed constant and further connected to said amplifier for controlling the gain thereof.
 2. The telephone network protective coupler recited in claim 1 wherein the limiting means further includes;gain control means connected to the amplifier and to the attenuator for controlling the gain of the amplifier.
 3. The telephone network protective coupler recited in claim 2, wherein the controlled gain amplifier includes:a feedback network having a controlled resistance element therein, the gain of the amplifier being dependent on the resistance of the controlled resistance element.
 4. The telephone network protective coupler recited in claim 3 wherein the maximum power control means includes:amplitude comparing means connected to the rectifier and to the attenuator for comparing the amplitude of the rectified a.c. signals to the output voltage of the attenuator; and means connected to the controlled resistance element and operated by the amplitude comparing means for controlling the resistance of the controlled resistance element to keep the ratio of the amplitude of the rectified a.c. signals to the output voltage of the attenuator less than or substantially equal to a fixed constant.
 5. The telephone network protective coupler recited in claim 3 wherein the maximum power control means includes:amplitude comparing means connected to the rectifier and to the attenuator for comparing the amplitude of the rectified a.c. signals to the output voltage of the attenuator; and means connected to the controlled resistance element and operated by the amplitude comparing means for controlling the resistance of the controlled resistance element to keep the power delivered to the telephone line in dB. relative to 1 milliwatt less than or substantially equal to the power loss of the attenuator.
 6. The telephone network protective coupler recited in claim 4 wherein the amplitude comparing means comprises an error amplifier.
 7. The switched network attachment device recited in claim 4 wherein the controlled resistance element comprises a photoconductive cell coupled to a lamp; andthe resistance controlling means comprises a lamp driver.
 8. The telephone network protective coupler recited in claim 2 wherein the controlled gain amplifier includes:a feedback network having a controlled resistance element therein, the gain of the amplifier being dependent on the resistance of the controlled resistance element.
 9. The telephone network protective coupler recited in claim 8 wherein the gain control means includes:gain comparing means connected to the output of the amplifier and to the attenuator for comparing a signal indicative of the gain of the amplifier to a signal indicative of the attenuation of the attenuator; and means connected to the controlled resistance element and operated by the gain comparing means for controlling the resistance of the controlled resistance element to keep the ratio of the gain of the amplifier to the attenuation of the attenuator constant.
 10. The telephone network protective coupler recited in claim 9 wherein the gain comparing means comprises an error amplifier.
 11. The telephone network protective coupler recited in claim 9 wherein the controlled resistance element comprises a photoconductive cell coupled to a lamp; andthe resistance controlling means comprises a lamp driver.
 12. The telephone network protective coupler recited in claim 1 wherein the maximum power control means is responsive to adjustment of the attenuator for keeping the power delivered to the telephone line in dB. relative to 1 milliwatt less than or substantially equal to the power loss of the attenuator.
 13. The switched network attachment device recited in claim 1 wherein the attenuator includes:a resistance network.
 14. The telephone network protective coupler recited in claim 1 including:a noise filter coupled between the attenuator and the gain control means.
 15. A method for limiting the signal power transmitted to a telephone central office by terminal equipment attached to a telephone line,comprising the steps of: connecting the terminal equipment to the telephone line by means of a four-wire transmission network which includes noncoincident receive and transmit paths for respectively receiving a.c. signals from and transmitting a.c. signals to the central office; incorporating a controlled gain amplifier in the transmit path for varying the attenuation of a.c. signals transmitted to the telephone central office; disposing externally of the transmit path an attenuator connected to a d.c. voltage source; setting to a predetermined value that attenuation of the attenuator; rectifying externally of the transmit path the a.c. signals after passage through the amplifier; and keeping the ratio of the amplitude of the rectified a.c. signals to the output voltage of the attenuator less than or substantially equal to a fixed constant by controlling the gain of said amplifier.
 16. The method recited in claim 15 including the steps of:controlling the gain of amplifier in response to the setting of the attenuator.
 17. The method recited in claim 15 including the step of: keeping the power output of the amplifier in dB. relative to 1 milliwatt less than or substantially equal to the power loss of the attenuator in response to adjustment of the attenuator.
 18. The method recited in claim 15 including the step of:filtering noise coupled into the attenuator.
 19. A method for limiting the signal power transmitted to a telephone network central office by terminal equipment attached to a telephone line,the method comprising the steps of: connecting the terminal equipment to the telephone line by means of a four-wire transmission network which includes noncoincident receive and transmit paths for respectively receiving a.c. signals from and transmitting a.c. signals to the central office; incorporating in the transmit path for varying the attenuation of a.c. signals transmitted to the telephone central office a controlled gain amplifier which includes a feedback network having a controlled resistance element therein, the gain of the amplifier being dependent on the resistance of the controlled resistance element; disposing externally of the transmit path an attenuator connected to a d.c. voltage source; setting to a predetermined value the attenuation of the attenuator; rectifying externally of the transmit path the a.c. signals after passage through the amplifier; and controlling the resistance of the controlled resistance element to keep the ratio of the amplitude of the rectified a.c. signals to the output voltage of the attenuator less than or substantially equal to a fixed constant.
 20. The method recited in claim 19 including the steps of:controlling the resistance of the controlled resistance element to keep the ratio of the gain of the amplifier to the attenuation of the attenuator constant.
 21. The method recited in claim 19 including the steps of:comparing the amplitude of the rectified a.c. signals to the output voltage of the attenuator; and controlling the resistance of the controlled resistance element to keep the power output of the amplifier in dB. relative to 1 milliwatt less than or substantially equal to the power loss of the attenuator.
 22. The method recited in claim 19 including the step of:filtering noise coupled into the attenuator.
 23. A telephone network protective coupler for connect-terminal equipment to a central office by means of a telephone line comprising:a four-wire transmission network including noncoincident receive and transmit paths for respectively receiving a.c. signals from and transmitting a.c. signals to the central office, a controlled gain amplifier connected in the transmit path for varying the attenuation of a.c. signals transmitted to the central office; an attenuator disposed externally of the transmit path and having an attenuation set to a predetermined value; means for connecting the attenuator to a d.c. voltage source; a rectifier disposed externally of the transmit path for rectifying a.c. signals after passage through the amplifier; and maximum power control means connected to the rectifier and to the attenuator for keeping the ratio of the amplitude of the rectified a.c. signals to the output voltage of the attenuator less than or substantially equal to a fixed constant and further connected to said amplifier for controlling the gain thereof.
 24. The telephone network protective coupler recited in claim 23 further including:gain control means connected to the amplifier and to the attenuator for controlling the gain of the amplifier.
 25. The telephone network protective coupler recited in claim 24 wherein the controlled gain amplifier includes:a feedback network having a controlled resistance element therein, the gain of the amplifier being dependent on the resistance of the controlled resistance element.
 26. The telephone network protective coupler recited in claim 25 wherein the maximum power control means includes:amplitude comparing means connected to the rectifier and to the attenuator for comparing the amplitude of the rectified a.c. signals to the output voltage of the attenuator; and means connected to the controlled resistance element and operated by the amplitude comparing means for controlling the resistance of the controlled resistance element to keep the ratio of the amplitude of the rectified a.c. signals to the output voltage of the attenuator less than or substantially equal to a fixed constant.
 27. The telephone network protective coupler recited in claim 25 wherein the maximum power control means includes:amplitude comparing means connected to the rectifier and to the attenuator for comparing the amplitude of the rectified a.c. signals to the output voltage of the attenuator; and means connected to the controlled resistance element and operated by the amplitude comparing means for controlling the resistance of the controlled resistance element to keep the power output of the amplifier in dB. relative to 1 milliwatt less than or substantially equal to the power loss of the attenuator.
 28. The telephone network protective coupler recited in claim 26 wherein the amplitude comparing means comprises an error amplifier.
 29. The telephone network protective coupler recited in claim 26, wherein the controlled resistance element comprises a photoconductive cell coupled to a lamp; andthe resistance controlling means comprises a lamp driver.
 30. The telephone network protective coupler recited in claim 24 wherein the controlled gain amplifier includes:a feedback network having a controlled resistance element therein, the gain of the amplifier being dependent on the resistance of the controlled resistance element.
 31. The telephone network protective coupler recited in claim 30 wherein the gain control means includes:gain comparing means connected to the controlled gain amplifier and to the attenuator for comparing a signal indicative of the gain of the amplifier to a signal indicative of the attenuation of the attenuator; and means connected to the controlled resistance element and operated by the gain comparing means for controlling the resistance of the controlled resistance element to keep the ratio of the gain of the amplifier to the attenuation of the attenuator constant.
 32. The telephone network protective coupler recited in claim 31 wherein the gain comparing means comprises an error amplifier.
 33. The telephone network protective coupler recited in claim 31 wherein the controlled resistance element comprises a photoconductive cell coupled to a lamp; andthe resistance controlling means comprises a lamp driver.
 34. The telephone network protective coupler recited in claim 23 wherein the maximum power control means is responsive to adjustment of the attenuator for keeping the power output of the amplifier in dB. relative to 1 milliwatt less than or substantially equal to the power loss of the attenuator.
 35. The telephone network protective coupler recited in claim 23 wherein the attenuator includes:a resistance network.
 36. The telephone network protective coupler recited in claim 23 including:a noise filter coupled between the attenuator and the gain control means. 